Thermodynamic re-assessment of the Mg–Gd binary system coupling the microstructure evolution during ageing process

Abstract According to the available experimental phase equilibrium relations, thermochemical properties and the experimentally observed ageing precipitation sequence from literatures, the thermodynamic re-modeling and re-assessment of the Mg–Gd binary system have been carried out by means of CALPHAD (CALculation of PHAse Diagram) technique. The metastable phases, including GP zones (D019-type) and Mg7Gd intermediate, are described by the models (Gd,Mg%)0.75(Gd%,Mg)0.25 and (Gd,Mg%)0.875(Gd%,Mg)0.125, respectively, in order to deal with the order-disorder transitions between the ordered phases GP zones (D019-type)/Mg7Gd intermediate and the disordered solid solution HCP_A3(Gd,Mg). With the considerations of the influences of elastic strain energies and interface energies on the thermodynamic driving forces, the effective driving forces for the precipitates to nucleate are calculated to match the ageing precipitation sequence, α(Mg)SSSS(supersaturated solid solution)→ GP zones (D019-type)→ β′(c-bco, Mg7Gd)→ β1(D03, Mg3Gd)→ β(fcc, Mg5Gd). A set of the self-consistent thermodynamic parameters has been obtained. The present calculations of the phase diagram, the thermochemical properties, and the precipitation driving forces of the typical Mg–Gd alloys with x(Gd) = 0.01819 and 0.0318 at 498 K, are not only well consistent with the reported experimental data but also in a satisfactory agreement with the precipitation sequence in the early stage of ageing observed by high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM).